Last week, when NASA and NOAA jointly announced that 2015 was the hottest year on record by a sizable margin, the news may have seemed purely factual in nature.

Yet it’s also politically resonant. Not only are this year’s crop of Democratic candidates much more gung-ho on climate change than their Republican rivals, but one leading Republican candidate, Texas Sen. Ted Cruz, has repeatedlyargued that based on data from satellites, there has been “no significant warming whatsoever for the last 18 years,” as he put it recently in New Hampshire. That raises the question of whether Cruz would accept what these two U.S. science agencies say, which is based on surface data rather than the measurements of satellites.

“The satellite and weather balloon data do not show 2015 as a record year,” said the senator’s communications director, Rachael Slobodien, when asked for Cruz’s reaction to the announcement of a record hot year.

“That 2015 may have been a record year in the surface data set, this in no way contradicts the Senator’s statements that there has been a pause in warming,” she continued in a statement to the Post. “Furthermore, even taking the surface data at face value, a nominal record is not especially significant. The important question is whether temperatures are rising as fast as the models say they are supposed to. And even the global warming alarmists acknowledge that they are not.”

Among the current field of Republican presidential contenders, skepticism of human-caused climate change is no rarity, but Cruz has arguably gone the farthest in actively making a technical counterargument and invoking data.

“It doesn’t matter, it could be a combination of Stephen Hawking, Einstein, and the Pope, and it’s not going to move him off his talking point,” says David Titley, a retired Navy rear admiral and a meteorology professor at Penn State University, who countered Cruz during the December Senate hearing and defended the idea that the globe is indeed warming.

Here’s a recent MSNBC Hardball segment showing Cruz making this claim about the satellite data:

Why was 2015 the hottest year ever? Chris Mooney of the Washington Post explains.http://on.msnbc.com/1WBWHLr

So what’s going on with this claim that satellite data — rather than the surface data that NASA and NOAA used to proclaim the 2015 record year — don’t show any warming?

Cruz’s claims actually harken back to a longstanding debate in the climate sphere, one dating at least back to 1990. That’s when Roy Spencer and John Christy, two satellite experts affiliated with NASA and the University of Alabama in Huntsville, argued in the prominent journal Science that satellite measurements are able to deliver “more precise atmospheric temperature information than that obtained from the relatively sparse distribution of thermometers over the earth’s surface.”

The satellite data in question are derived from polar orbiting satellites that carry microwave sounding units that can measure the microwave emissions given off by oxygen molecules in the atmosphere. They don’t measure just the planet’s surface, but the entire troposphere (the layer of the atmosphere where weather happens, extending up to about 6 miles) and above it.

And over the years, these data have been often cited to call into question whether the globe is warming as fast as surface temperature readings would seem to suggest.

The University of Alabama in Huntsville’s Christy — often dubbed a climate change “skeptic,” though he says he agrees that “humans do affect the climate” — was a witness at Cruz’s December hearing, where his testimony criticized the “failure of the scientific community to objectively approach the study of climate and climate change.” Christy also argued that satellite measurements of the troposphere show less warming than climate change models would predict, thus calling those models into question.

Reached for an interview, Christy reiterated this idea — that the overall satellite record, which dates back to 1979, doesn’t show as much warming as climate models would predict. “There’s something weird here, there’s a discrepancy in our understanding of how the system is supposed to respond to greenhouse gases,” he said. In his statements about satellites and about climate models, it appears that Cruz is relying at least in part on Christy as a key expert.

But other scientists who focus on the satellite data, such as physicist Carl Mears of Remote Sensing Systems in Santa Rosa, Ca., take a significantly different tack. Mears agrees the satellite record shows less warming than models expect in recent years, but he also emphasizes potential problems and uncertainties with the satellite data.

“Part of Sen. Cruz’s argument is that the satellite data is more accurate than the surface data. We don’t think that’s true,” Mears said.

Mears and Benjamin Santer, a climate scientist at Lawrence Livermore National Laboratory, recently co-authored a strong critique of Cruz’s “Data or Dogma” hearing. “Satellites are not a thermometer in space, they’re not making direct measurements of atmospheric temperature, they’re measuring the microwave emissions from oxygen molecules,” Santer said. He cites numerous types of uncertainty associated with satellite temperature data and numerous corrections to it required — such as due to satellites’ orbital drifts — making the entire endeavor a “tough job.”

“There’s over a dozen satellites that you need to string together and each of them have calibration and drift issues that need to be dealt with,” added Gavin Schmidt, director of the Goddard Institute for Space Studies at NASA. “If there’s an issue with any particular satellite or any particular calibration it affects all the temperatures, so it’s much easier to have systematic issues that affect the whole record.”

No warming in 18 years?

Warts and all, the satellite datasets are the basis for Cruz’s claim that there has been little or no warming lately — or, as the senator says, in the past 18 years.

If you only consider satellite data — and on top of that, if you grant Cruz the particular time window that he’s chosen and start the analysis around the very warm El Niño event of 1997/1998 – then Mears concurs that recent warming in the satellite record is quite limited.

“In the most recent decade and a half, the satellite data show maybe a little bit of warming, or maybe no warming, and that’s considerably less than the surface record shows,” he said.

“But,” he continued, “if you look over the longer period, the satellite data show enough warming that you can’t possibly explain it without using the forcing caused by the increase in CO2 and methane and so forth.”

The full satellite record goes back to the year 1979. Mears sent the Post an analysis of the warming trends in satellite data in his dataset, both from 1979 through 2015 but also if you set the starting point of the analysis at 1996, 1997, 1998, 1999, and 2000.

The upshot was clear. For the lower troposphere, for instance, there is a considerably greater warming trend if you start in 1979 than in any of the later years. The trend is in fact negative in Mears’ dataset if you start in 1998. But if you take the entire record, then the trend is 0.123 degrees Celsius per decade. (You can see for yourself here.)

“That enormous El Niño is right at the beginning of their time period, and that tends to make the trends less than they would be if you picked the other year,” says Mears.

When asked whether it’s fair to only consider 18 years of the satellite record, Christy noted that the “real world” produced a relative dearth of warming in the satellite record over the past 18 years, and Cruz is simply describing that. “The data is telling you that that’s the period of time that doesn’t have significant warming,” he said. He added, though, that “I like to start in 1979, in the beginning.”

How El Niño years reverberate in the troposphere

When announcing the 2015 temperature record, NOAA noted that 2015 was only the third warmest year on record for the lower troposphere, according to both the University of Alabama in Huntsville’s data and that produced by RSS. The middle troposphere — two to six miles into the atmosphere — was either the third or fourth warmest in 2015, depending on whether you cite the University of Alabama-Huntsville dataset or RSS, the agency said.

The warmest year for the troposphere in bothrecords remains 1998 — the second year of a very hot 97-98 El Niño event. And that, say scientists, is because El Niño years produce particularly pronounced spikes in atmospheric temperatures – with a time lag.

This image obtained November 16, 2015 from the National Oceanic and Atmospheric Administration (NOAA) shows the satellite sea surface temperature departure for the month of October 2015, where orange-red colors are above normal temperatures and are indicative of El Niño. (AFP PHOTO / HANDOUT / NOAA)

“In the atmosphere, higher up, the response to events is different than at the surface,” said Thomas Karl, head of NOAA’s National Centers for Environmental Information, which announced the 2015 temperature record along with NASA. “In the upper parts of the atmosphere there’s a delayed response to any warming due to El Niño, it’s usually the subsequent year that it becomes much more sensitive, usually it responds about 50 percent greater than the surface.” For this reason, Karl says 2016 could show a very warm or record year for the satellite record — and Mears agrees.

So, for that matter, does Christy. “I wouldn’t be surprised if 2016 would be the hottest in the satellite record,” he said.

The strong warming of the troposphere during El Niños, Santer said, is a key reason why it’s misleading to start a trend analysis with a year like 1998.

“It’s impermissible to cherry pick,” he said. “You can’t do that. You need to look at all 18 year periods of record….and you get a very different opinion and perception of the reality of warming.”

Where we live

Surveying all of this, it’s clear that if you wanted to find an argument to raise doubt about global warming, then the claim that satellites show little or no warming in the past 18 years is not a bad choice.

Nonetheless, the rebuttal that this constitutes cherry picking — first, choosing a favored dataset, and then on top of that, choosing a favorable time interval too — is hard to counter. It simply isn’t a full presentation of the evidence.

There’s also a question of relevance. True, satellites have a more universal view of the Earth than do ground-based readings, which are only taken in specific locations and can be very limited in some areas, like the Arctic or Antarctic. Yet the temperature at the surface is what we most care about.

“We don’t live up in the middle of the troposphere. The only time we’re up there, we’re in a sealed container called an airplane,” said Katharine Hayhoe, a climate scientist at Texas Tech University.

The surface record is also far longer — dating back to 1880 — and as of the end of 2015 shows a full 1 degree Celsius of warming over pre-industrial temperatures, according to NASA and NOAA. That’s halfway toward the 2 degree marker that the world agreed, in the recent Paris climate accord, to avoid. It is a major planetary change.

All of these are indicators of changes in planetary temperature. And it is surveying the totality of all of this evidence that makes many scientists support the consensus view on climate change, even if the satellite record may still hold mysteries that need explaining.

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Chris MooneyChris Mooney covers climate change, energy, and the environment. He has reported from the 2015 Paris climate negotiations, the Northwest Passage, and the Greenland ice sheet, among other locations, and has written four books about science, politics and climate change. Follow